Still now, agricultural and horticultural production is seriously damaged by diseases, and there exists a factor of chemical resistance acquired by microorganisms and fungi against existing chemicals. Thus, it has been desired to develop novel plant disease control agents for agricultural and horticultural use. Therefore, many agents for controlling plant diseases have so far been investigated and developed and have been used according to application methods adapted for respective chemicals. In recent years, various labor-saving methods of applying chemicals have been required with the advance of age of farmers, and reduction in amount of chemical to be used has been required for the purpose of preserving earth environment. Hence, developments of plant disease control agents for agricultural and horticultural use and methods of using them which can meet the requirements have strongly been demanded. As one of using methods of applying a plant disease control agent for agricultural and horticultural use, there has been known a method of applying the agent to seeds of objective plants or to a cultivation carrier for sowing objective plants. This method has the advantage that, since it requires the chemical to exist only around the seeds of the objective plant or around the cultivation carrier for sowing the objective plant, it leads to reduction of the amount of chemicals to be used, serves to reduce the load to earth environment and, further, reduces contact between workers and chemicals so as to increase safety of workers and leads to saving of works.
Under such circumstances, certain kinds of 1,2,3-thiadiazole compounds are described to be useful as plant disease control agents for agricultural and horticultural use (for example, see patent document 1). However, there has been neither description nor suggestion about use of the 4-cyclopropyl-1,2,3-thiadiazole compounds of the invention represented by formula (I) as agents for controlling plant diseases or methods of applying the controlling agents to seeds of objective plants or to a cultivation carrier for sowing the objective plant. Also, there is no specific description about the 4-cyclopropyl-1,2,3-thiadiazole compounds of the invention represented by formula (I). On the other hand, there has been described a method of applying specific 1,2,3-thiadiazole compounds to seeds of an objective plant or to a cultivation carrier for sowing an objective plant (for example, see patent document 2). In patent document 2, however, there is only one compound as Example and, with regard to 4-cyclopropane compounds, although two compounds are described in the compound list, there are no Examples demonstrating that the compounds are actually effective, and there are absolutely no disclosure in the document about the specifically excellent effect provided by using the 4-cyclopropyl-1,2,3-thiadiazole compounds of the invention represented by formula (I) according to the present method of use.                Patent document 1: JP-A-8-325110        Patent document 2: JP-A-2001-10909        